Implicit Parameters: Dynamic Scoping with Static Types (Extended Abstract)

This paper introduces a language extension, called implicit parameters, for Hindley-Milner-typed programming languages that provides dynamically scoped variables within a statically typed framework. Implicit parameters are lexically distinct from regular identifiers, and are bound by a special with construct whose scope is dynamic, rather than static as with let. Implicit parameters are treated by the type system as parameters that are not explicitly declared, but are inferred from their use. We present implicit parameters using a small Hindley-Milner calculus. We give a type system, a type inference algorithm, and several semantics. In addition, we have also built an experimental extension to the Hugs 98 Haskell interpreter that implements implicit parameters [3]. We use this to present several examples